In rechargeable electrochemical cells, weight and portability are important considerations. It is also advantageous for rechargeable cells to have long operating lives without the necessity of periodic maintenance. Rechargeable electrochemical cells are used in numerous consumer devices such as calculators, portable radios, and cellular phones. They are often configured into a sealed power pack that is designed as an integral part of a specific device. Rechargeable electrochemical cells can also be configured as larger "cell packs" or "battery packs".
Rechargeable electrochemical cells may be classified as "nonaqueous" cells or "aqueous" cells. An example of a nonaqueous electrochemical cell is a lithium-ion cell which uses intercalation compounds for both anode and cathode, and a liquid organic or polymer electrolyte. Aqueous electrochemical cells may be classified as either "acidic" or "alkaline". An example of an acidic electrochemical cell is a lead-acid cell which uses lead dioxide as the active material of the positive electrode and metallic lead, in a high-surface area porous structure, as the negative active material. Examples of alkaline electrochemical cells are nickel cadmium cells (Ni--Cd) and nickel-metal hydride cells (Ni--MH). Ni--MH cells use negative electrodes having a hydrogen absorbing alloy as the active material. The hydrogen absorbing alloy is capable of the reversible electrochemical storage of hydrogen. Ni--MH cells typically use a positive electrode having nickel hydroxide as the active material. The negative and positive electrodes are spaced apart in an alkaline electrolyte such as potassium hydroxide.
Upon application of an electrical potential across a Ni--MH cell, the hydrogen absorbing alloy active material of the negative electrode is charged by the electrochemical absorption of hydrogen and the electrochemical discharge of a hydroxyl ion, forming a metal hydride. This is shown in equation (1): ##EQU1##
The negative electrode reactions are reversible. Upon discharge, the stored hydrogen is released from the metal hydride to form a water molecule and release an electron.
Hydrogen absorbing alloys called "Ovonic" alloys result from tailoring the local chemical order and local structural order by the incorporation of selected modifier elements into a host matrix. Disordered hydrogen absorbing alloys have a substantially increased density of catalytically active sites and storage sites compared to single or multi-phase crystalline materials. These additional sites are responsible for improved efficiency of electrochemical charging/discharging and an increase in electrical energy storage capacity. The nature and number of storage sites can even be designed independently of the catalytically active sites. More specifically, these alloys are tailored to allow bulk storage of the dissociated hydrogen atoms at bonding strengths within the range of reversibility suitable for use in secondary battery applications.
Some extremely efficient electrochemical hydrogen storage alloys were formulated, based on the disordered materials described above. These are the Ti--V--Zr--Ni type active materials such as disclosed in U.S. Pat. No. 4,551,400 ("the '400 Patent") the disclosure of which is incorporated herein by reference. These materials reversibly form hydrides in order to store hydrogen. All the materials used in the '400 Patent utilize a generic Ti--V--Ni composition, where at least Ti, V, and Ni are present and may be modified with Cr, Zr, and Al. The materials of the '400 Patent are multiphase materials, which may contain, but are not limited to, one or more phases with C.sub.14 and C.sub.15 type crystal structures.
Other Ti--V--Zr--Ni alloys, also used for rechargeable hydrogen storage negative electrodes, are described in U.S. Pat. No. 4,728,586 ("the '586 Patent"), the contents of which is incorporated herein by reference. The '586 Patent describes a specific sub-class of Ti--V--Ni--Zr alloys comprising Ti, V, Zr, Ni, and a fifth component, Cr. The '586 Patent, mentions the possibility of additives and modifiers beyond the Ti, V, Zr, Ni, and Cr components of the alloys, and generally discusses specific additives and modifiers, the amounts and interactions of these modifiers, and the particular benefits that could be expected from them. Other hydrogen absorbing alloy materials are discussed in U.S. Pat. Nos. 5,096,667, 5,135,589, 5,277,999, 5,238,756, 5,407,761, and 5,536,591, the contents of which are incorporated herein by reference.
The hydrogen storage alloy negative electrode may be paste type or non-paste type. Non-paste type electrodes are formed by pressing or compacting the active hydrogen absorbing alloy onto a conductive substrate. A method of fabricating non-paste types negative electrodes is disclosed in U.S. Pat. No. 4,820,481 ("the '481 Patent") the disclosure of which is incorporated herein by reference.
As described in the '481 Patent, a hydrogen storage alloy powder is loaded into a loss in weight feeder. The powder is released from the feeder and passes, through a feeder hose, onto a vibrating chute assembly. The powder is vibrated along the chute assembly to a material divider which is adapted to distribute the active electrode material in an even, uniform manner onto a moving continuous web of substrate material. After the active material is distributed onto the substrate surface, it is compacted onto the substrate by a rolling mill. It is noted that the distribution of active material onto the substrate is important to electrode performance since performance is directly related to the uniformity of both the density and thickness of the active material.
In the specific method described by the '481 Patent, the newly formed continuous web of electrode material (i.e., substrate with compacted active material) is sintered to increase its mechanical durability and cycle life. Sintering promotes both the particle-to-particle bonding of the active material as well as the bonding of the active material particles to the substrate.
Sintering is an expensive and time-consuming step in the electrode fabrication process. To reduce the need for sintering, a binder may be added to the hydrogen alloy powder to increase the durability and cycle life of the electrode. U.S. Pat. No. 5,753,386 (the '386 Patent), U.S. Pat. No. 5,707,763 (the '763 Patent), and U.S. Pat. No. 5,393,617 (the '617 Patent) all disclose using binders with active electrode materials. The contents of U.S. Pat. Nos. 5,753,386, 5,707,763, and 5,393,617 are incorporated by reference herein.
The binders disclosed by the '386, '763, and '617 Patents are "fibrillating" binders comprising, at least in part, a fibrillating component. The '386 Patent describes a non-paste type hydrogen absorbing alloy electrode. The active material is made by combining a hydrogen absorbing alloy powder and a polymer binder. As stated on column 12, lines 40-47 of the '386 Patent: "When . . . the nonpaste type hydrogen absorbing-alloy electrode . . . is made, polytetrafluoroethylene (PTFE) is preferably used as the polymer binder because PTFE is made to fibers by being stirred . . . ." The '763 Patent discloses a binder having both a fibrillating component (a core) and a nonfibrillating component (a shell) where the ratio by weight of the fibrillating to nonfibrillating components (core-to-shell ratio) is between 98:2 to 50:50. The '617 Patent describes a mixture of a mischmetal hydride alloy, and a TEFLON powder which "was fibrillated to form a fibrous, lace-like network . . . " (column 10, lines 24-26 of the '617 Patent).
A fibrillating polymer fibrillates when mixed or stirred, developing a fibrous, lace-like, agglomerated consistency. The composition formed by mixing a fibrillating binder with an active material thus has a consistency which cannot be used with an electrode fabrication process like the one described by the '481 Patent. The agglomerated, gummy consistency hinders (or even totally blocks) the flow of the material through the feeder, powder chute and material divider, thereby preventing uniform delivery to the substrate.
A new active electrode composition is needed which has a consistency that can be used with an electrode fabrication process like the one described in the '481 Patent, and which includes a polymeric binder for increased strength, durability and cycle life.